Most household or office electrical circuits, particularly those related to lighting, typically utilize mechanical switches. These mechanical switches may be of the contact type for on/off activation, or include a means for varying the power supplied to the circuit, i.e., to perform a “dimming” function, to variably alter power supplied to a light or to act as a speed control for a fan.
Non-contact switches that rely on a change in capacitance to perform an on/off function have been proposed. These devices sense the presence or absence of an object in front of the switch by the change in capacitance.
In U.S. Pat. No. 5,973,608, a non-contact switching system is described that utilizes selected components to provide on/off and dimming functions. However, the components in one embodiment are preferably housed at a centralized location, requiring dedicated wiring from the sensors to the central controller and then back to the activated circuits.
In addition, the dimming function is achieved in defined steps which require particular components for each step, further increasing costs and complexity. For example, the '608 patent uses outputs of a capacitive sensor at predetermined levels to activate different stepped levels of dimmer output. This means that the number of capacitive sensor outputs is proportional to possible dimmer levels. A digital value representing those levels is passed through a programmable logic device (PLD) and then latched. Latch output determines output power level. This means that the number of power control outputs is proportional to the output power levels. To construct a smooth dimmer, small increments in output power level are needed, which requires a proportionally high number of power control outputs, and thus multiple large components with a high number of pin counts (PLD, latch, clock chips). Even if a CPU were considered for use in the device, the input pin count would still be equal to the sensor output number and the output pin count equal to the latch output number of the CPU, which would still be proportional to the number of stepped dimmer levels, requiring a large CPU chip and numerous interconnections and peripheral components in the circuit.
Control of the '608 switch is hand movement dependent. To switch the light on, the hand must be moved from the furthest zone into the closest zone of the sensor. To switch the light off, the hand must be pulled from the closest zone into the furthest zone of the sensor. Clearly two different types of movement are needed for basic operation of the switch. To users unfamiliar with the device this could result in confusion.
In U.S. Pat. No. 5,716,129, a non-contact switch includes an oscillator having a frequency output that varies with proximity of a hand. The components are intended for insertion into a lamp base or an ornamental shell. The component count and/or component size are quite large and would not fit into a standard wall box, as is clear from the view showing these components in a lamp base, and this is without a dimmer control circuit. The device is clearly not capable of functioning as a direct replacement of a mechanical wall mounted switch. The device also requires both a neutral and a live connection to the AC power source, while in many wall boxes and circuit designs, only one lead is accessible, rendering such a device useless as a direct replacement.
A particular problem with the prior art is the inability to provide a direct replacement for a mechanical switch. For example, a direct replacement of a mechanical on/off toggle switch must be capable of fitting within a space defined by a common electrical box. Utilizing special size boxes or special wiring adds substantially to the cost of installation, and is prohibitive in any retrofit application.
In U.S. Pat. No. 6,750,564, a compact non-contact electrical switch, having the same inventor as the present application, was presented which addressed these problems. This is an improvement on the switch disclosed therein.
However, it was found that the switches described in the previous patent were somewhat difficult to mount close together in a gang installation. If they are too close, the sensor ranges may overlap and then it becomes difficult to activate just a single switch without activating neighboring switches by accident. While this problem can be overcome by reducing the sensor ranges, this may also reduce the usability of the switch.
Another problem that may occur in a ganged installation relates to the capacitive sensors. If the capacitive sensors in adjacent switches operate as oscillators, pulse generators or any other changing voltage/current method that measures sensors impedance, they can interfere with each other. Basically each capacitive sensor is an oscillator that radiates electromagnetic waves into the surrounding space. If the sensors are close together, and their frequency is similar, they can “tune” to each other, just as a radio tunes to a transmitter frequency, and when that occurs, the sensors can interfere so the switches won't work correctly. While this can be overcome by reducing the range of the sensor or by shielding the sensor field, both solutions may degrade somewhat switch usability.